The invention relates to an integrable current supply circuit for feeding a supply current to a signal line.
Terminals which are connected to a signal line are in many applications supplied via the signal line with a supply current as the power supply for the terminal. This supply current is in this case produced by a current supply circuit, which is connected to the signal line.
EP 0 836 286 A1 describes a circuit arrangement for obtaining a supply voltage from a bus line on which message signals are superimposed on an operating voltage. The circuit arrangement contains a constant current source and what is referred to as a diplexer, which is actuated when message pulses occur. A circuit section is provided which provides the diplexer element with a control current without influencing the constant current source.
DE 195 10 279 A1 describes a current source power supply with a current mirror circuit having a high input impedance, and with a current which flows in a first and a second current path.- Furthermore, the current source power supply contains a current reference device and a current sensing device, as well as a feedback circuit.
U.S. Pat. No. 5,535,243 describes a transmitter power supply circuit, which receives a loop current from a control loop, and supplies a measurement circuit with current.
FIG. 1 shows a signal line to which a terminal which has the impedance ZL is connected. Signals, that is to say voice or data signals, are transmitted via the line and are emitted by a signal driver circuit. The signal driver circuit receives the signals to be transmitted from a current source. The supply current for the terminal is fed in at a node K. The supply current is produced by a conventional current source. In this case, the current source contains a voltage source, whose output is connected to an inductance L. The impedance of the inductance L is in this case designed such that, even at high frequencies, the output impedance of the current source is greater than a predetermined minimum impedance which is specified, for example, in CCITT Standard I.430 as an impedance template to be complied with.
The disadvantage of the current source according to the prior art illustrated in FIG. 1, for feeding a supply current into the signal line, is that the inductance or inductor L cannot be integrated and must be connected to an integrated circuit as an external component.
The current supply circuit illustrated in FIG. 2 has thus been proposed in the prior art.
The conventional supply circuit shown in FIG. 3 comprises a current source IQ, whose source current is amplified by a transistor T and is emitted as the supply current Isupply via the output A of the current supply circuit to the feed node K. The operating point of the transistor T is in this case set by means of a current source I0.
The disadvantage of the conventional current supply circuit illustrated in FIG. 2 is that the transistor T, which acts as a current amplifier, and the current source IQ have parasitic capacitances, which are indicated by dashed lines as the parasitic capacitor Cpar in FIG. 2. The parasitic capacitances that are contained in the current supply circuit severely reduce the output impedance Zout of the current supply circuit at high frequencies, so that the minimum current source impedances specified by the Standard are not complied with.
FIG. 3 shows the profile of the output impedance of the conventional current supply circuit illustrated in FIG. 2. As can be seen from FIG. 3, the impedance template to be complied with in accordance with CCITT I1.430 is considerably undershot above a frequency of about 60 kHz.
The object of the present invention is therefore to provide an integrable current supply circuit which has a high output impedance Zout even at high frequencies.
According to the invention, this object is achieved by an integrable current supply circuit having the features specified in patent claim 1.
The invention provides an integrable current supply circuit for feeding a supply current to a signal line having a current source for producing a source current which is emitted via a connection line to an input of a current amplifier, which amplifies the source current and feeds the amplified source current as a supply current via an output of the current supply line to the signal line, with the current source and the current amplifier each having parasitic capacitances, with the integrable current supply circuit according to the invention furthermore having a compensation capacitor which is connected to the output of the current supply circuit and whose capacitance corresponds to the parasitic capacitance, and a current mirror circuit which emits the charging current that flows through the compensation capacitor in order to compensate for the charging currents flowing through the parasitic capacitances in mirrored form onto the connection line connected between the current source and the current amplifier.
The magnitude of the source current is preferably adjustable.
The current source is preferably a pnp bipolar transistor or a PMOS transistor.
The operating point of the current amplifier is preferably adjustable.
In one preferred embodiment of the integrable current supply circuit, the current mirror circuit comprises two npn bipolar transistors, whose base connections are connected to one another.
In an alternative embodiment, the current mirror circuit comprises two NMOS transistors, whose gate connections are connected to one another.
The current mirror circuit of the integrable current supply circuit according to the invention preferably mirrors the current flowing through the parasitic capacitances with a predetermined current mirror ratio, and emits the mirrored current in inverted form to the connection line.
In one preferred embodiment, the current mirror ratio of the current mirror circuit is minus one.
In an alternative embodiment, the current/mirror ratio is equal to the ratio of the parasitic capacitances to the capacitance of the compensation capacitor.
The quiescent current of the current mirror circuit is preferably adjustable.
In one particularly preferred embodiment of the integrable current supply circuit, this circuit is designed differentially.
The output impedance of the current supply circuit according to the invention is preferably high over a broad frequency range from 1 kHz to 1 MHz, and is greater than a predetermined minimum impedance value.
Preferred embodiments of the integrable current supply circuit according to the invention will be described in the following text with reference to the attached figures in order to explain features that are essential to the invention.